330 research outputs found
Conformal mapping of ultrasonic crystals: confining ultrasound and cochlear-like wave guiding
Conformal mapping of a slab of a two-dimensional ultrasonic crystal generate
a closed geometrical arrangement of ultrasonic scatterers with appealing
acoustic properties. This acoustic shell is able to confine ultrasonic modes.
Some of these internal resonances can be induced from an external wave source.
The mapping of a linear defect produces a wave-guide that exhibits a
spatial-frequency selection analogous to that characteristic of a synthetic
"cochlea". Both, experimental and theoretical results are reported here.Comment: 4 pages, 4 figure
The dynamics of cultivation and floods in arable lands of Central Argentina
Although floods in watersheds have been associated with land-use change since ancient times, the dynamics of flooding is still incompletely understood. In this paper we explored the relations between rainfall, groundwater level, and cultivation to explain the dynamics of floods in the extremely flat and valuable arable lands of the Quinto river watershed, in central Argentina. The analysis involved an area of 12.4 million hectare during a 26-year period (1978â2003), which comprised two extensive flooding episodes in 1983â1988 and 1996â2003. Supported by information from surveys as well as field and remote sensing measurements, we explored the correlation among precipitation, groundwater levels, flooded area and land use. Flood extension was associated to the dynamics of groundwater level. While no correlation with rainfall was recorded in lowlands, a significant correlation (<i>P</i>&lt;0.01) between groundwater and rainfall in highlands was found when estimations comprise a time lag of one year. Correlations between groundwater level and flood extension were positive in all cases, but while highly significant relations (<i>P</i>&lt;0.01) were found in highlands, non significant relations (<i>P</i>&gt;0.05) predominate in lowlands. Our analysis supports the existence of a cyclic mechanism driven by the reciprocal influence between cultivation and groundwater in highlands. This cycle would involve the following stages: (a) cultivation boosts the elevation of groundwater levels through decreased evapotranspiration; (b) as groundwater level rises, floods spread causing a decline of land cultivation; (c) flooding propitiates higher evapotranspiration favouring its own retraction; (d) cultivation expands again following the retreat of floods. Thus, cultivation would trigger a destabilizing feedback self affecting future cultivation in the highlands. It is unlikely that such sequence can work in lowlands. The results suggest that rather than responding directly and solely to the same mechanism, floods in lowlands may be the combined result of various factors like local rainfall, groundwater level fluctuations, surface and subsurface lateral flow, and water-body interlinking. Although the hypothetical mechanisms proposed here require additional understanding efforts, they suggest a promising avenue of environmental management in which cultivation could be steered in the region to smooth the undesirable impacts of floods
RamĂłn Muntaner [Texto impreso] : cronista dels reys de AragĂł ciutadĂ de Valencia
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The Void Galaxy Survey
The Void Galaxy Survey (VGS) is a multi-wavelength program to study 60
void galaxies. Each has been selected from the deepest interior regions of
identified voids in the SDSS redshift survey on the basis of a unique geometric
technique, with no a prior selection of intrinsic properties of the void
galaxies. The project intends to study in detail the gas content, star
formation history and stellar content, as well as kinematics and dynamics of
void galaxies and their companions in a broad sample of void environments. It
involves the HI imaging of the gas distribution in each of the VGS galaxies.
Amongst its most tantalizing findings is the possible evidence for cold gas
accretion in some of the most interesting objects, amongst which are a polar
ring galaxy and a filamentary configuration of void galaxies. Here we shortly
describe the scope of the VGS and the results of the full analysis of the pilot
sample of 15 void galaxies.Comment: 9 pages, 6 figures. This is an extended version of a paper to appear
in "Environment and the Formation of Galaxies: 30 years later", Proceedings
of Symposium 2 of JENAM 2010, eds. I. Ferreras, A. Pasquali, ASSP, Springer.
Version with highres figures at
http://www.astro.rug.nl/~weygaert/vgs_jenam_weygaert.col.pd
The environmental dependence of the structure of outer galactic discs in STAGES spiral galaxies
We present an analysis of V-band radial surface brightness profiles for
spiral galaxies from the field and cluster environments using Hubble Space
Telescope/Advanced Camera for Surveys imaging and data from the Space Telescope
A901/2 Galaxy Evolution Survey (STAGES). We use a large sample of ~330 face-on
to intermediately inclined spiral galaxies and assess the effect of the galaxy
environment on the azimuthally averaged radial surface brightness mu profiles
for each galaxy in the outer stellar disc (24 < mu < 26.5 mag per sq arcsec).
For galaxies with a purely exponential outer disc (~50 per cent), we determine
the significance of an environmental dependence on the outer disc scalelength
h_out. For galaxies with a broken exponential in their outer disc, either
down-bending (truncation, ~10 per cent) or up-bending (anti-truncation, ~40 per
cent), we measure the strength T (outer-to-inner scalelength ratio,
log_10(h_out/h_in) of the mu breaks and determine the significance of an
environmental dependence on break strength T. Surprisingly, we find no evidence
to suggest any such environmental dependence on either outer disc scalelength
h_out or break strength T, implying that the galaxy environment is not
affecting the stellar distribution in the outer stellar disc. We also find that
for galaxies with small effective radii (r_e < 3 kpc) there is a lack of outer
disc truncations in both the field and cluster environments. Our results
suggest that the stellar distribution in the outer disc of spiral galaxies is
not significantly affected by the galaxy environment.Comment: Accepted to MNRAS. Appendix A available at
http://www.nottingham.ac.uk/~ppxdtm/STAGES_profiles_appendix.pd
Structural, morphological, and magnetic characterizations of (FexMn1-x)2O3 nanocrystals: A comprehensive stoichiometric determination
Iron manganese trioxide (FexMn1-x)2O3 nanocrystals were synthesized by the
sol-gel method. The 80 K Mossbauer spectrum was well-fitted using two doublets
representing the 8b and 24d crystallographic sites of the (FexMn1-x)2O3 phase
and two weak extra sextets which were attributed to crystalline and amorphous
hematite. Our findings showed formation of a bixbyite primary phase. The Raman
spectrum exhibits six Raman active modes, typical of (Fe,Mn)2O3, and two extra
Raman modes associated with the secondary hematite phase. X-ray photoelectron
spectroscopy analysis confirmed the presence of oxygen vacancy onto the
(FexMn1-x)2O3 particle surface, with varying oxidation states. X-band magnetic
resonance data revealed a single broad resonance line in the whole temperature
range (3.8 K - 300 K). The temperature dependence of both resonance field and
resonance linewidth shows a remarkable change in the range of 40 - 50 K, herein
credited to surface spin glass behavior. The model picture used assumes
(FexMn1-x)2O3 nanoparticles with a core-shell structure. Results indicate that
below about 50 K the spin system of shell reveals a paramagnetic to spin
glass-like transition upon cooling, with a critical temperature estimated at 43
K. In the higher temperature range, the superparamagnetic hematite (secondary)
phase contributes remarkably to the temperature dependence of the resonance
linewidth. Zero-field-cooled (ZFC) and fieldcooled (FC) data show strong
irreversibility and a peak in the ZFC curve at 33 K, attributed to a
paramagnetic-ferrimagnetic transition of the main phase. Hysteresis curve at 5
K shows a low coercive field of 4 kOe, with the magnetization not reaching
saturation at 70 kOe, suggesting the occurrence of a ferrimagnetic core with a
magnetic disorder at surface, characteristic of core-shell spin-glass-like
behavior
The SAURON project. II. Sample and early results
Early results are reported from the SAURON survey of the kinematics and
stellar populations of a representative sample of nearby E, S0 and Sa galaxies.
The survey is aimed at determining the intrinsic shape of the galaxies, their
orbital structure, the mass-to-light ratio as a function of radius, the age and
metallicity of the stellar populations, and the frequency of kinematically
decoupled cores and nuclear black holes. The construction of the representative
sample is described, and its properties are illustrated. A comparison with
long-slit spectroscopic data establishes that the SAURON measurements are
comparable to, or better than, the highest-quality determinations. Comparisons
are presented for NGC 3384 and NGC 4365 where stellar velocities and velocity
dispersions are determined to a precision of 6 km/s, and the h3 and h4
parameters of the line-of-sight velocity distribution to a precision of better
than 0.02. Extraction of accurate gas emission-line intensities, velocities and
line widths from the datacubes is illustrated for NGC 5813. Comparisons with
published line-strengths for NGC 3384 and NGC 5813 reveal uncertainties of <
0.1 A on the measurements of the Hbeta, Mgb and Fe5270 indices. Integral-field
mapping uniquely connects measurements of the kinematics and stellar
populations to the galaxy morphology. The maps presented here illustrate the
rich stellar kinematics, gaseous kinematics, and line-strength distributions of
early-type galaxies. The results include the discovery of a thin, edge-on, disk
in NGC 3623, confirm the axisymmetric shape of the central region of M32,
illustrate the LINER nucleus and surrounding counter-rotating star-forming ring
in NGC 7742, and suggest a uniform stellar population in the decoupled core
galaxy NGC 5813.Comment: 20 pages, 17 figures. To be published in MNRAS. Version with full
resolution images available at
http://www.strw.leidenuniv.nl/~dynamics/Instruments/Sauron/pub_list.htm
A New Automatic Method to Identify Galaxy Mergers I. Description and Application to the STAGES Survey
We present an automatic method to identify galaxy mergers using the
morphological information contained in the residual images of galaxies after
the subtraction of a Sersic model. The removal of the bulk signal from the host
galaxy light is done with the aim of detecting the fainter minor mergers. The
specific morphological parameters that are used in the merger diagnostic
suggested here are the Residual Flux Fraction and the asymmetry of the
residuals. The new diagnostic has been calibrated and optimized so that the
resulting merger sample is very complete. However, the contamination by
non-mergers is also high. If the same optimization method is adopted for
combinations of other structural parameters such as the CAS system, the merger
indicator we introduce yields merger samples of equal or higher statistical
quality than the samples obtained through the use of other structural
parameters. We explore the ability of the method presented here to select minor
mergers by identifying a sample of visually classified mergers that would not
have been picked up by the use of the CAS system, when using its usual limits.
Given the low prevalence of mergers among the general population of galaxies
and the optimization used here, we find that the merger diagnostic introduced
in this work is best used as a negative merger test, i.e., it is very effective
at selecting non-merging galaxies. As with all the currently available
automatic methods, the sample of merger candidates selected is contaminated by
non-mergers, and further steps are needed to produce a clean sample. This
merger diagnostic has been developed using the HST/ACS F606W images of the
A901/02 cluster (z=0.165) obtained by the STAGES team. In particular, we have
focused on a mass and magnitude limited sample (log M/M_{O}>9.0,
R_{Vega}<23.5mag)) which includes 905 cluster galaxies and 655 field galaxies
of all morphological types.Comment: 25 pages, 14 figures, 4 tables. To appear in MNRA
Frequency and properties of bars in cluster and field galaxies at intermediate redshifts
We present a study of large-scale bars in field and cluster environments out
to redshifts of ~0.8 using a final sample of 945 moderately inclined disk
galaxies drawn from the EDisCS project. We characterize bars and their host
galaxies and look for relations between the presence of a bar and the
properties of the underlying disk. We investigate whether the fraction and
properties of bars in clusters are different from their counterparts in the
field. The total optical bar fraction in the redshift range z=0.4-0.8 (median
z=0.60), averaged over the entire sample, is 25% (20% for strong bars). For the
cluster and field subsamples, we measure bar fractions of 24% and 29%,
respectively. We find that bars in clusters are on average longer than in the
field and preferentially found close to the cluster center, where the bar
fraction is somewhat higher (~31%) than at larger distances (~18%). These
findings however rely on a relatively small subsample and might be affected by
small number statistics. In agreement with local studies, we find that
disk-dominated galaxies have a higher optical bar fraction (~45%) than
bulge-dominated galaxies (~15%). This result is based on Hubble types and
effective radii and does not change with redshift. The latter finding implies
that bar formation or dissolution is strongly connected to the emergence of the
morphological structure of a disk and is typically accompanied by a transition
in the Hubble type. (abridged)Comment: 17 pages, accepted for publication in A&
Variations in Amazonian forest canopy structure and light environments across environmental and disturbance gradients.
A critical problem in tropical forest ecology is understanding how vegetation structure and function vary over environmental gradients. The degree to which forest structure changes across the Amazon basin and the role of environmental variability in shaping forest structure and dynamics are poorly characterised, despite the importance of these forests for regional and global climate. To address these challenges, we connected 10 years of investigations to amass a large database of ground-based profiling canopy lidar (PCL) data from 297 Amazon forest plots across large-scale environmental and disturbance gradients. Mean annual precipitation varied from 1,963 to 3,159 mm, number of dry season months from 0 to 5, and plot soil types covered about half of the variation in phosphorus, exchangeable cation, and soil physical property values observed in Amazonia. We quantified detailed metrics of vertical and horizontal structure and canopy light environments. Forest structure varied considerably across plots; maximum canopy height ranged from 6.1 to 35.7 m, gap fraction from 0.00 to 0.36, LAI from 0.5 to 7.3, rugosity from 1.5 to 7.5 m, and the relative height of 50% light transmission from 0.3 to 0.8. Disturbed sites exhibited almost twice the level of variation (SD) to non-disturbed sites for many metrics. Vertical leaf area density (LAD) profiles also showed high between plot variability, especially at low and high relative canopy heights. Plots with similar LAD profiles sometimes exhibited different distributions of ?canopy photic environment layers??where canopy leaf area is separated into photic environment layers by depth from canopy surface. This demonstrates that LAD profiles alone are insufficient for characterising canopy environments, essential to light-driven regeneration and carbon cycle processes. In addition, we evaluated relationships between lidar metrics and environmental variables extracted from geospatial layers. Our dataset allows a unique and detailed multi-site analysis of canopy structure and environments across the Amazon, including regions with little or no lidar sampling. Examining how structural attributes alter across environmental gradients is critical to understanding how current and future climate influences Amazonian forest structure, function, and dynamics.Paper 499657
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